1. Field of the Invention
The present invention relates to a plasmon antenna for a thermally assisted magnetic head used in a magnetic recording device such as a hard disk drive.
2. Related Background Art
Thermally assisted magnetic recording has been the object of ongoing research in recent years with a view to enhancing magnetic recording density. To write information in thermally assisted magnetic recording, a magnetic head applies a magnetic field on a region to be recorded on, in the magnetic recording medium, while the region is heated. The plasmon antennas used for such heating exhibit plasmon resonance, generating near-field light as a result, when irradiated by a laser beam. Specifically, a laser beam propagates through an optical waveguide and strikes a plasmon antenna provided at the leading end of the optical waveguide.
The laser beam comprises a TM (transverse magnetic) polarized component and a TE (Transverse electric) polarized component. To operate the plasmon antenna with a TM-polarized component, two conceivable methods are 1) a method in which a laser beam from a TM-polarized laser light source strikes the plasmon antenna directly, and 2) a method in which a laser beam from a TE-polarized laser light source is converted to TM polarization via a polarization mode converter, and strikes then the plasmon antenna. When the leading end vertex of a triangular antenna is disposed in the track running direction of the magnetic recording medium, the orientation of a perpendicular line drawn from the vertex of the triangle to the base thereof preferably coincides with the TM mode direction.
In the former method 1), however, it is difficult to manufacture good TM-polarized laser light sources, while energy conversion efficiency is poor. Such a method is thus inadequate for near-field light generation. The latter method 2) utilizes a polarization mode converter, which is disadvantageous both in terms of cost and of transmission loss during polarization mode conversion. Moreover, the conversion efficiency of the polarization mode converter is of about 80% at best. The development of plasmon antennas that work with TE polarization holds thus great promise.
Conventional plasmon antennas are disclosed in, for instance, U.S. Pat. No. 5,696,372, 7,177,236 or 6,649,894, which disclose bow-tie plasmon antennas. The plan-view shape of a conventional bow-tie plasmon antenna comprises a set of small metal films, shaped as isosceles triangles, wherein the vertices of the small metal films oppose each other with a gap in between In plasmon antennas having such a structure, near-field light is generated at the position of the gap.
The structure of conventional plasmon antennas, however, may preclude carrying out sufficient writing. Specifically, the inventors have found, as a result of diligent research, that in a conventional structure the position of a magnetic pole that applies a write magnetic field is spaced apart from the position of the gap at which near-field light is generated, which is problematic owing to the difficulty of applying the write magnetic field before the magnetic recording medium cools down after having been heated by the near-field light.